Hallo
Ok, so puff up is what all chickens do when they feel bad.
Gradual diseases are plenty, but with ducks involved I'll venture a guess.
Dirty or muddied water causes several diseases.
So, go check if the water is dirty.
If it is replace and if you can, move the ducks. Ducks soak their feed in water. I always make sure that the chickens can't step into their water or scratch dirt into it.
Slow and gradual weight loss is probably bacterial. Like
e.coli or coccidiosis.
So, that's tetracycline or a sulfa medicine in the water for several days ( read the bottle )
If you can afford it
Baytril 5% injections.
Airway infections is something you can listen for, sit by them and listen if they are gasping for air. Having difficulty getting air into the windpipe sounds like asthma. If this is the case I'll go for the Baytril. If they are inoculated for Infectious Bronchitus then you have less to worry about. If not, then this could be a slow end to 80% of your chickens.
Now that your chickens are already sick
upgrading the food is a good idea. Commercial Boiler grower ( puts meat on the bones) can help already thin sick chickens survive. Clean water that you would drink. You can add some
probiotics to the water. I live on a farm and have to purify my chickens drinking water.
Then I would suggest you read the thread on fermented foods, for small flock owners keeping disease at bay is better. Also there are a lot of things people add to the water. AVC, probiotics, multi vitamins.
If you're lucky then it's just bacteria. Easy to cure. Good luck.